An important component of a disposable absorbent article such as a diaper is an absorbent core including water-absorbing polymer particles. These water-absorbing polymer particles ensure that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness. Especially useful water-absorbing polymer particles are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g. sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like.
These water-absorbing polymer particles need to have adequately high absorption capacity, as well as adequately high gel strength. Absorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. At the same time, the water-absorbing polymer particles need to have a good permeability for fluid transport through the swollen crosslinked polymers network.
The properties of water-absorbing polymer particles have been characterized in various ways. The absorbent capacity (CRC) in grams of liquid per gram of water-absorbing polymer particles has been used, as well as the absorption speed as measured by the Free Swell Rate (FSR) and their permeability as measured by the Urine Permeability Measurement (UPM) test.
Low molecular weight polymer chains that are not incorporated into the crosslinked polymer network exist in the water-absorbing polymer particles and are called “extractables”. These chains can be extracted from the crosslinked polymer network when the water-absorbing polymer particles are swollen in excess liquid. The “extractables” do not contribute to the water-absorbing polymer particles performance.
Moreover, the “extractables” increase the ionic strength in the environment of the water-absorbing polymer particles due to their charged groups and negatively impact the absorption capacity of the water-absorbing polymer particles. Therefore, the quantity of “extractables” is important in determining the optimum performance of the water-absorbing polymer particles. There is a need to provide water-absorbing polymer particles with a low quantity of “extractables”.
Water-absorbing polymer particles with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure and decrease the number of “extractables”. However, increasing the level of internal crosslinking or surface crosslinking typically reduce the absorption capacity of the water-absorbing polymer particles.
On the contrary, decreasing the level of internal crosslinking or surface crosslinking in water-absorbing polymer particles leads to high absorption capacity of the water-absorbing polymer particles but also to a high number of “extractables” and a relatively low permeability.
Therefore, there is a need to provide water-absorbing polymer particles that presents a right balance between having high absorption capacity, high permeability and a low number of “extractables”.
It is desirable to find a method for making water-absorbing polymer particles with improved performances in terms of capacity and permeability while having a low quantity of “extractables”.